Cardiovascular surgery is practiced with many forms of myocardial protection. The need for myocardial protection during such surgery is caused by the medical practice of draining the venous blood from the heart into an extracorporeal oxygenator, oxygenating the blood, and returning the oxygenated blood to the heart downstream of the coronary arteries; that is, the heart muscle itself is denied the benefit of oxygenated blood during many of these surgeries. This of course risks ischemic damage to the heart muscle.
One common form of myocardial protection is the use of cardioplegia solution to cause temporary paralysis of the heart. This elective stopping of cardiac activity is known to be accomplished by the injection of chemicals, selective hypothermia, electrical stimuli or a combination of these techniques. A typical combination is the injection of a cold cardioplegia solution directly into the heart circulation. Known cold cardioplegia solutions include "clear" cardioplegia, "crystalloid" cardioplegia and "blood" cardioplegia.
In the case of each of these solutions, the solution at the time of the surgery is pumped from a source through an administration set, sometimes called a tube set, and injected into the patient's heart after being suitably cooled and debubbled. A typical pump for such applications is known as a roller pump. This is a form of positive displacement pump that will develop considerable head pressures and resultant patient delivery pressures that may be medically unacceptable and capable of damaging the tube set in the case, for example, of an obstruction downstream of the pump. Partially because of this capability, these administration sets are carefully moderated to control the delivery pressure to the heart, and sometimes are provided with weaker portions in the form of pop-off valves. These pumps are typically turned off when the attending medical personnel do not desire the delivery of the cooled and debubbled cardioplegia solution into the patient's heart. However, when the pump is turned off, much of the cardioplegia solution within the tube set is not being cooled. This allows rewarming of the solution to ambient conditions and partial loss of the therapeutic hypothermia.